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Breathing Underwater: The Science Of Diving

Without the air in our atmosphere, there would be no life on the planet.

Most people don’t give a passing thought to the air that they breathe other than perhaps worrying about how polluted it is.

Commercial diving, on the other hand, is an industry that is obsessed with the science of breathing underwater.

The Air That We Breathe

While the composition of the atmosphere on our planet has not always been the same, it has remained relatively stable for the last 1.4 billion years or so.

This allowed life to evolve.

Earth’s atmosphere is comprised of a number of different gasses. However, air is comprised primarily of Nitrogen (78.08%) and Oxygen (20.95%). The remainder is composed of gasses such as Aragon, Carbon Dioxide, and Helium, etc.

Given that humans evolved under these conditions, it is hardly surprising that our bodies have  gotten kind of used to breathing these gasses within a relatively small range of variation.

To understand how serious this is, consider this:

If you were to hitch a lift to the top of Mount Everest, where the oxygen level is roughly 33% of 

what it is at sea level, you would be dead within a matter of minutes.  

Oxygen level is one example of how a change in the air we breathe can have dire consequences for our bodies. 

When it comes to breathing underwater, nitrogen, helium, and factors such as pressure are all things that commercial divers must pay very careful attention to if they want to survive. 

Under Pressure

At ambient pressures, the gasses in our atmosphere at or around sea level are optimum for the human body.

However, underwater things change quite dramatically.

As humans dive deeper and deeper, the ambient pressure affecting their bodies increases.

To compensate, the lungs become more pressurized to match the pressure around them. This leads to a diver breathing more and more gas per breath as they go deeper.

This means that at 50m, for example,  where the pressure is about 5 times greater on the surface, the lungs will contain 5 times the amount of gasses.

One side effect of this is that the lungs absorb more and more nitrogen which then passes into the bloodstream.

DIT Instructor, Emil Bruesch, showing students how to use the Hyperbaric Chamber.

If a diver ascends too quickly, the nitrogen absorbed into the solution in their bloodstream forms bubbles. Known as the “bends” or Nitrogen Narcosis, this process can cause anything from itchy skin to death.

Fortunately, there are things divers can do to prevent this from happening.

Types of gases for breathing underwater

By altering the composition of the air that divers breathe, it is possible to allow them to go deeper and deeper.

0 – 30 meters

Most scuba divers only venture to a maximum depth of 30m. At these relatively shallow depths, the pressures are relatively low.

This doesn’t mean that divers don’t need to be cautious. Scuba divers are still required to make “safety” or decompression stops before they return to the surface.

30 – 50 meters

While most divers use a normal air mix, another precaution is the use of Nitrox. This is a gas mix that reduces the amount of nitrogen that divers breathe by replacing it with oxygen. 

Nitrox allows for an increased decompression limit, meaning divers can stay under for longer.

While Nitrox is a good solution for use at shallower depths, the increased oxygen at increased pressures can lead to oxygen toxicity which affects the central nervous system.

50 – 100 meters

To overcome this, the gas mixture must be altered again to reduce both the amount of nitrogen and oxygen. 

These gasses are replaced by the inert gas helium which doesn’t cause any side effects in humans other than making a diver sound like a chipmunk!

This gas mix is known as Trimix.

Role of helium in deep diving

Helium is what is known as a “Noble Gas”. This means that it has a full configuration of electrons in its outer shell and therefore doesn’t react with other substances.

This is great news for divers as it means that helium is non-toxic to the human body at higher pressures. Consequently, divers don’t need to worry about side effects as with the case of nitrogen narcosis and oxygen toxicity.

Because helium doesn’t cause any tissue damage even after months of use, it is ideal for use with deep dives required in saturation diving.

100 meters and beyond

Heliox is created when the nitrogen in the mix is completely replaced by helium. This gas mixture is used when divers are working at around 100 to 150 meters.

However, breathing such large volumes of helium at depths below 150m can lead to a condition known as “High-Pressure Nervous Syndrome”. This condition can cause such symptoms such as nausea, dizziness, and decreased mental performance, etc.

To combat this, an additional gas, hydrogen, can be added to the Heliox mix to create Hydreliox. 

Another option is removing the helium altogether to create a mixture known as Hydrox.

Both mixtures have been proven to allow divers to go as deep as an incredible 700m, where few divers have ever gone before!

Aran Davis, Writer for Water Welders

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